The present invention relates to packaging machines, and more particularly relates to sealers used in horizontal form, fill, and seal packaging machines.
Machines that utilize Radio Frequency as the means for welding (RF Welders) are known in the art. RF welders are typically used for sealing and embossing appliqué on RF sealable materials. Such materials are commonly used in processing materials such as PVC, PU, PET, PETG and polyolefin. The welders process these materials in manufacturing, for example, vinyl envelopes and binders having internal pockets. For example, pockets are sealed to the binder on first and second side edges and a bottom edge, leaving a top edge open for egress. RF welding of the edges obviates the need for stitches.
The theory and implementation of RF welding is disclosed in U.S. Pat. No. 5,833,915, incorporated herein by reference. FIG. 2 discloses an RF welder 1 known in the art. A standard generator (not shown) provides power to the welder 1 at an FCC mandated frequency of 27.12 MHz, using standard 50 Ohm coaxial cable 2. The coaxial cable is used because it is an excellent conveyor of energy and suffers very little loss. The term load in this application should be understood to mean all components seen as a load with respect to the generator.
Referencing FIG. 2, the welder 1 has a top plate 3 and a bottom plate 4 that are used as electrodes for transferring electrical energy through a subject material 5 and die 6, where the die 6 has impressions 6a used for embossing or welding.
The die 6 is attached to the top plate 3 and acts as an electrode in tandem with the top plate 3. The die 6 has conductive electrical characteristics which alter the load characteristics. The material 5 is non-metallic and acts as a dielectric, absorbing energy passed between the top and bottom plates 3 and 4, to emboss or weld the material 5. The dielectric characteristics of the material 5 also alter the electrical characteristics of the load.
Accordingly, the impedance of the load is a combination of the impedance of all of the components in the electrical conduction loop plus the material being processed. Since the impedance varies from part to part and during the welding process, optimum power is not delivered to the weld with a manually fixed impedance match. If the matching network or the frequency generator automatically adjust the reactance of the load to maintain the correct impedance at the generator, maximum power is always delivered to the weld during processing giving a better, quicker and more efficient weld.
If not enough energy is delivered to the load the material 5 may not weld or fail to become embossed. If too much energy passes through the load, the material may burn and other load components may fail (such as the coaxial cable which can be over-dissipated).
If the frequency at which energy is delivered to the material is incorrect, the welding or embossing of that material will suffer lagging or leading, which is known to provide poor quality results. More specifically, the power that transfers through the fabric may rise continuously through the weld cycle, or the power may rise to a maximum value and fall as the die sinks into the material. Such a power fluctuation provides an uneven weld with potential undesirable results in, for example, weld strength or emboss appearance.
Accordingly, with differing load impedance characteristics, a result of changing load impedance, there may be a slow reaction by the fabric causing a slow start of the welding or embossing or a complete failure to weld or emboss the material. Other problems include flashing caused by a high voltage arc-over.
As a result of the unique impedance characteristic, the RF welder must be electrically tuned, via impedance matching, after placement of the die 6 and material 5 within the welder. After the tuning of the load impedance, the power delivered will be optimum and over-dissipation of the cables and other elements will not occur. Energy will be passed through the platen and fabric at frequency of 27.12 MHz, preventing lagging or leading of the welding or embossing process.
Normally, impedance matching occurs as illustrated in FIG. 4. As indicated, the die and material must first be installed on the machine at S1 and S2. The user is capable of adjusting the load impedance by manually adjusting a capacitor external to the generator at S3. The capacitor is adjusted by attempting to weld a material while adjusting capacitor electrodes towards or away from a sample dielectric. The RF welder 1 is calibrated by running the RF welder 1 and checking the material at S4 and S5 to determine if the quality is satisfactory at S6.
When the material welds appropriately, it is deduced that the RF welder 1 is tuned properly, the power is set correctly, and over-dissipation does not occur. Once the RF welder 1 is tuned, a series of identical materials can be welded or embossed at steps S7 and S8.
Various problems normally occur with the manual adjusting of the capacitor. First, it is relatively impossible for a person to adjust the capacitance so that the generator sees exactly 50 ohms due to the inherent sensitivity and robustness of the RF welder 1. Rather, manual adjusting typically provides at least a 5% error on the frequency adjustment. Also, a sample does not capture the dielectric characteristic for a series of materials since each individual piece of material has unique inconsistencies which affect the electrical characteristics of each weld. To adjust for these problems, the RF welder 1 must constantly be checked for quality at S9. Further, if non-identical materials or dies are used, then the RF welder 1 must be continuously retuned.
Even if the impedance is adjusted for each unique material 5, the material capacitance tends to change as the die sinks into the material. As the die sinks the capacitance changes, impedance changes, and optimum power is not transferred to the die and material. Accordingly, the falloff causes a decrease in the ability for the die to weld or emboss the material.